Syrian forces beat up a prominent Syrian political cartoonist and left him bleeding on the side of a road, in the latest episode of a campaign to quash dissent against the regime of President Bashar al-Assad.

Ali Ferzat, 60, is one of the Arab world's most famous cultural figures, and his drawings have pushed at the boundaries of freedom of expression in Syria.

The attack on Ferzat came as the Iranian leader, Mahmoud Ahmadinejad, called for a dialogue between Assad and the opposition to bring a peaceful end to the protests.

Working from a gallery in central Damascus, Ferzat has long criticised the bureaucracy and corruption of the Syrian and other Arab regimes – earning him a death threat from former Iraqi president Saddam Hussein. Since March he has turned to depicting the uprising.

In the early hours of Thursday, masked men seized Ferzat from the street and forced him in to a van. A relative has said that Ferzat's attackers targeted his hands, breaking them both, and told him it was "just a warning" before leaving him by the roadside with a bag over his head.

In a galvanising moment similar to when the corpse of 13-year-old Hamza al-Khateeb was returned to his parents bearing marks of severe torture in May, Syrians have been expressing outrage. Messages have circulated online and some Facebook users changed their profile picture to a photograph of Ferzat in hospital.

The dissident artist, who once described himself as having a friendship with Assad, warned in 2007 of an impending "monumental crisis" if the regime did not reform. He has since become increasingly critical of the regime and its brutal crackdown.

In a recent cartoon he criticised the regime's offers of reforms, with a picture of an official with rosebuds in his speech bubble – and a turd in his head.

Assad has shrugged off international condemnation and continues to use security forces and thugs to kill and arrest his opponents. Arrests and raids continued across the country on Thursday with at least five people shot dead across the country and tanks sent into al-Boukamal on the Iraqi border, activists said.

Until now Ahmadinejad has refused to intervene in Syria, a regional ally it fears losing should the Assad regime fall. But in a rare public statement about the uprising, he told a Lebanese TV station: "What happened in Libya is what the west wants to happen to Syria too ... for this reason the Syrian people and the government should be conscious and try to solve their problems together and do their reforms themselves."

At least 2,200 people have been killed since mid-March, says the UN. On Wednesday, the EU imposed sanctions against Iran's elite Quds Force, saying it is helping crush the revolt.

Outspoken cultural figures have in the past been able to get away with more criticism than others. But in recent weeks, several artists, writers and actors have been arrested.

Last month, Ibrahim al-Qashoush, the composer of a popular anti-regime song in Hama, was found dead with his vocal chords removed.

"At this stage, fame may be more of a danger than a protection because the regime does not want any prominent figure to come to the fore and provide a public face for the revolution," said Ammar Abdulhamid, a US-based dissident and son of Syrian actor Mona Wasif.

Ferzat was born in Hama, where a tank assault on the eve of Ramadan to reinstate government control prompted widespread outrage. A graduate of Damascus University's faculty of fine arts, his initial work in the 1970s appeared in state-run newspapers. By 1980 his cartoons were being published in the French newspaper Le Monde, earning global recognition, exhibitions and prizes.

In a 2001 interview with the Guardian, Ferzat recalled that before becoming president, Assad visited one of his exhibitions and said that some of the cartoons banned in Syria should have been published. He also published satirical paper al-Domar (Lamplighter), which ran from late 2000 until he was forced to close it in 2003.